1st Punic war
Cause and inducement
Rome now had Italy south of Gallia Cisalpina (plain of the river PO) under control and quit peaceful. We'll try and understand why the Romans and the Carthaginians, who had concluded 4 treaties of friendship (Polybius says 3), got into 3 terrible wars.
The Romans in this time were trained and experienced soldiers. The battlefield was a place where honour could be obtained. Wealth was measured by the land you owned. Once the Romans set out to subdue a people they would,'t rest until they succeeded. After a lost battle they simply started all over again, but never it meant an end of the war they were waging. After a won battle they would be willing to make peace, but always at the expense of the other. A lot of income was earned on the battle field or conquered cities, trade was seen as an activity unworthy for members of the senate. The territory they now controlled would provide them with all the means they needed among those, soldiers.
Carthago traditionally founded in 815 bc, was initially a trading post of Tyrus (Tyre, Lebanon), so the people living there were Phoenicians. Their main source of income was trade. The Phoenicians had always sailed the Mediterranean sea and founded trading posts like Gades in Spain Utica (near Carthago) and Lpqy (later Lepcis Magna). Like the Romans originally a kingdom (queen Dido) but also in later days replaced by an olicharchy. Here the thought was: who is smart enough to make a lot of money is also good en ought to rule. Carthaginians didn't really like war: a captain of a merchant vessel earns money the captain of a war ship only costs money. This doesn't mean they didn't wage a war when necessary. They had stopped to recruit their own people for an army. They would rent mercenaries whenever needed. After a lost battle they would try to make peace for a reasonable price. But at sea they were the masters second to none, that is until the Romans decided to defeat them there.
Carthago's power was at least equal to Rome's. Carthago was still a city state, that controlled the territory around it and the eastern half of Sicily. Carthago had tried time and again to conquer all of Sicily, but every time it found Syracusae in it's way. They probably had in mind the grain
they needed to feed the people back home, but Rome, thinking like a general, saw threat: If Carthago would conquer all of Sicily they would be very close to Roman territory, too close for comfort. When the Romans saw a good opportunity, they decided to act.
A just war?
As always the Romans would try and justify their actions. The Carthaginians would have started the war, because a Carthaginian fleet had neared Tarentum 8(!) years earlier. Even Polybius admits there was no hostility and when they had learned the Romans were there, they left. This means this was no justification for this war.
Could the Roman, without violating the treaties with Carthago attack the Carthaginian forces, that were besieging Messana? Polybius uses a lot of words to try and make clear they could, because in the treaties it would be forbidden only in the territories controlled by Carthago and Carthago did not control Messana. This may be true or not, the Romans violated the treaties at the moment they attacked the Carthaginian forces, because Messana wasn't a Roman city either.
A necessary war?
If Carthago would attack the Italian coasts or not is less important than the Roman perception. What the Romans saw was a powerful city expanding towards Rome. All the powers they had defeated before had done exactly that and sooner or later attacked Rome. The senate chose to prevent that attack this time. I think it probable the Romans induced the Mamertines to ask for Roman help. And the Mamertines did.